During a braking application for a vehicle, the body of the vehicle tilts to shift a major portion of the weight of the vehicle to the front wheels. Therefore, the traction of the rear wheel is reduced so that if equal pressures are communicated to the front and rear wheels, the rear wheels will skid or lock up to create an unsafe driving condition. In order to prevent lockup for the rear wheels the prior art provides a proportioning valve to restrict communication of pressurized fluid to the rear wheels in response to increasing brake fluid pressure.
Moreover, the prior art provides an inertia sensing mass which generates a force in response to vehicle deceleration. The force of the inertia sensing mass is transmitted to the proportioning valve to modify the restriction of pressurized fluid to the rear wheels. The inertia sensing mass is provided to account for the loaded condition of the vehicle, as a loaded vehicle will experience less deceleration than an unloaded vehicle, given one level of pressurized brake fluid.
In prior art inertia sensing proportioning valves, the inertia sensing mass is generally disposed within the fluid path connecting the master cylinder with the wheel cylinder, or movable to impart positive movement to the proportioning valve assembly. Consequently, the inertia sensing mass is subjected to changes in the fluid flow, temperature and viscosity of the brake fluid.